This is how I understood it to be... not all of the nucleotides are replicated at the end of the chromosome, thus telomerase adds telomere (repeating nucleotides which code for essentially nothing) to existing chromosomes to prevent loss of genetic info after replication. As the chromosome is further replicated, more and more of the end is not replicated to an extent that the replicated chromosome's sequence at the end, which would have coded for a gene in the chromosome which it was replicated from, codes for a nonfunctional gene. When this happens, chromosomes can't replicate and cells undergoes apoptosis and hence aging. I could be wrong though hopefully others would clarify it more!
